Broadband wave guide waterload of the type employing a quarter wave window transformer



1970 F. o. JOHNSON 3,538,461

BROADBAND WAVE GUIDE WATERLOAD OF THE TYPE EMPLOYING A QUARTER WAVE WINDOW TRANSFORMER Filed May 22, 1968 FIG-I FIG.3

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5.5 610 6.5 TIT FREQUENCY (0H1) FIG. 5 5 Has N 2.0- INDUCTIVE IRIS a: PRESENT |.8- INVENTION uz- INT T /4 CERAMIC wfiInowANncooLANT I0 I 1' l v I I W 35 ABRUPT TRANSITION 4.9 5.5 51 III 6.5 6.9 To AN OVERSIZE GUIDE FREQUENCY (cc) [2- INVENTOR. FLOYD o. JOHNSON l BY 5.? 5.9 6.| 63 6.5 MFW FREQUENCY (Ge) ATTORNEY United States Patent U.S. Cl. 333-22 3 Claims ABSTRACT OF THE DISCLOSURE A broadband waveguide waterload is described. The waveguide waterload includes a section waveguide to be connected at one end to a source of microwave energy to be absorbed and sealed off at its other end by means of a microwave window member sealed across the end of the waveguide to provide a liquid impervious barrier thereacross. The closed end of the waveguide looks into a stream of dielectric lossy liquid coolant for absorbing microwave energy passing through the window from the source. The window member is made approximately an integral number of quarter electrical wavelengths thick to provide a quarter wave impedance transformer for matching the impedance of the waveguide to the impedance of the dielectric coolant. The waveguide includes an abrupt waveguide transition from a first section of the waveguide having a certain cross sectional dimension to a second section of the waveguide having a larger cross sectional dimension and the window member is sealed across the larger section of waveguide. The abrupt waveguide transition produces a wave reflective discontinuity for cancelling the wave reflection from the mismatch between the window member and the stream of dielectric coolant. Its wave reflection has a dispersive characteristic similar to that of the wave reflection from the window member such that a broadband waveguide load results. The abrupt waveguide transition is located with the range 0.375n)\ to 0.625n)\ from the face of the window member, where n is any integer value including zero and A the guide wavelength within the waveguide.

DESCRIPTION OF THE PRIOR ART Heretofore, waveguide waterloads of the type wherein a quarter wave microwave window was sealed over the end of a waveguide for matching the impedance of the liquid coolant, such as water, to the impedance of the waveguide have been built. It was discovered in these waterloads that the dielectric window transformer did not provide a perfect impedance match between the dielectric fluid and the waveguide over a wideband of frequencies and, therefore, additional impedance matching devices in the form of inductive irises and capacitive irises have been employed to produce a wave reflection for concelling the wave reflection from the interface between the window member and the dielectric liquid for broadbanding the load. Examples of loads of this type are disclosed and claimed in U.S. Pats. 3,360,750 issued Dec. 26, 1967 and U.S. 3,289,109 issued Nov. 29, 1966, both assigned to the same assignee as the present invention.

One of the problems associated with the prior art reactive matching irises for matching out the Wave reflection from the dielectric fluid is that the shape of dispersive characteristic of the matching iris differs from the shape of the dispersive characteristicof the wave reflection from the dielectric fluid. Therefore, the match is only valid over that part of the dispersive characteristics which have the same shape. It turns out that when the waterload is operated near the high end of the operating band of the waveguide, where the waveguide is less dispersive, substantial bandwidts on the order of 15% can be obtained. However, when the standard waveguide is operated near the low end of the passband of the waveguide, where the waveguide is more dispersive, the operating passband of the waterload is decreased substantially below 15% to on the order of 10%. Therefore, an improved matching device is required which will have a dispersive characteristic similar to that of the wave reflection from the dielectric fluid, whereby the passband of the waterload can be increased.

SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is the provision of an approved broadband waveguide waterload of the type employing a quarter wave window transformer.

One feature of the present invention is the provision, in a waveguide waterload of the type employing a quarter wave window transformer, of an abrupt waveguide transition from a first waveguide section to a larger waveguide section with the window being disposed in the larger section of waveguide, whereby the shape of the dispersion characteristic of the wave reflection from the abrupt transition is made similar to that of the wave reflection from the window 35 broadening passband of the Waveguide load.

Another feature of the present invention is the same as the preceding feature wherein the abrupt waveguide transition is disposed within the range of 0.375% to 0.625% from the face of the window member, where n is any integer value including zero and is the guide wavelength within the waveguide.

Another feature of the present invention is the same as any one or more of the preceding features wherein the abrupt waveguide transition is tapered for matching the magnitude of the wave reflection from the abrupt transition to the magnitude of the wave reflection from the window member to obtain a broadband impedance match.

Other features and advantages of the present invention become apparent upon perusal of the following specification taken in connection with the accompanying drawings wherein;

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinal sectional view of a waveguide waterload embodying features of the present invention, such view being taken looking down on the broad wall of the waveguide,

FIG. 2 is a sectional view of the structure of FIG. 1 taken along line 22 in the direction of the arrows.

FIG. 3 is a sectional view of the structure of FIG. 1 taken along line 33 in the direction of the arrows,

FIG. 4 is a plot of guide wavelength, in inches, versus frequency, in gigahertz, depicting the dispersive characteristics of twosizes of rectangular waveguide,

FIG. 5 is a plot of voltage standing wave ratio versus frequency in gigahertz depicting the magnitude of the wave reflection versus frequency for the discontinuity produced between the window and the dielectric fluid, the wave reflection produced by the prior art inductive matching iris, and by an abrupt waveguide transition of the persent invention, and

FIG. 6 is a plot of voltage standing wave ratio versus freqeuncy in gigahertz for the waveguide waterload of the present invention as compared with that of the prior art.

3 DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIGS. l-3, there is shown the wave guide waterload 1 of the present invention. The Waterload 1 includes a section of hollow rectangular waveguide 2 having a flange 3 at one end thereof for connecting the waveguide 2 to a source of microwave energy to be dissipated. A dielectric wave permeable microwave window member 4 is sealed across the other end of the waveguide 2 to provide liquid impervious barrier across the waveguide 2. In addition, the dielectric window member 4 is made approximately an odd integral number of quarter wavelengths thick in the direction of power flow therethrough to form an odd quarter wave impedance transformer section for matching the impedance of the waveguide 2, on the source side to the impedance of a coolant stream 5 flowing across the outer face 6 of the window 4.

The liquid coolant is a lossy dielectric liquid such as for example water or ethylene glycol or mixtures thereof. The coolant stream is contained within a V-shaped channel 7 formed in a metallic block '8, as of copper, brazed over the window end of the waveguide section 2. A pair of pipes 9 and 10 are sealed in fluid in communication with the end of the V-shaped channel 7 and on opposite sides of the block 8. Inlet pipe 10 has its longitudinal axis directed against the window such that the coolant stream 5 is directed against the window to provide maximum cooling for the window member 4. The stream 5, after absorbing the microwave energy and being heated in the process, exits from the load via output pipe 9.

The waveguide section 2 includes an abrupt waveguide transition 11 from a first section of waveguide 12 of standard height and breadth for the passband of the waterload 1 to a second section of waveguide 13, which latter section of guide is substantially larger than the standard waveguide section 12. The abrupt Waveguide transition 11 is provided to produce a wave reflection of the proper magnitude and phase to cancel a waverefiection from the interface of the dielectric fluid stream 5 with the face 6 of the window member 4 for broadbanding the passband of the waterload 1.

Referring now to FIG. 4 there is shown the guide wavelength in inches versus frequency in gigahertz for a standard waveguide cross-sectional size, such as WR137 and the guide wavelength versus frequency curve for a larger or oversize waveguide, namely, WR187. From a comparison of these two curves, it is seen that the larger waveguide, namely, the WRl87 is much less dispersive over the frequency band from 5.5 gigahertz to 7.0 gigahertz. Thus, it is desirable to place the window member 4 in a section of waveguide which is oversize for the passband of the waterload such that the dispersive characteristic of the wave reflection from the window member 4 is rendered less dispersive over the passband of interest to facilitate; broadband matching.

Referring now to FIG. 5 there is shown a plot of voltage standing wave ratio versus frequency, which depicts the magnitude of the reflection versus frequency for an inductive iris, for the interface between a quarter wave ceramic window and a coolant stream 5' and for an abrupt waveguide transition 11 to an oversize guide 13. From FIG. 5, it is seen that the shape of the dispersion curve for the abrupt waveguide transition 11 much more closely approximates the shape of the dispersion curve for the window-coolant interface 6 than does the dispersion curve for the inductive iris. Thus, it is expected and the results will show that use of the abrupt waveguide transition 11 provides a much broader band match than that obtained by the prior art inductive iris. Indeed this is the case as shown in FIG. 6.

The spacing S from he window 4 to the abrupt Waveguide transition 11 is chosen such that the phase of the reflection from the abrupt transition 11 has the opposite phase to that of the reflection produced by the window 4, such that the reflection from the abrupt waveguide transition 11 cancels the reflection from the window 4. This spacing S is easily chosen in practice by dimensioning guide 12 such that it will slide within the oversized section of guide 13 and merely sliding a section of guide 12 within the guide 13 until the broadest band match is obtained. The abrupt transition 11 is then fixed at this position. It turns out that the optimum spacing S falls within the range of 0.375% to 0.625n where n is any integer value including zero and A is the guide wavelength within guide 13.

The magnitude of the wave reflection produced by the abrupt waveguide transition 11 is adjusted by removing material from the sharp corner of the abrupt transition 11. The material may be removed by rounding the corners, as shown in FIG. 1, or as alternative material may be removed by beveling the corner. In either case the abrupt transition 11 is tapered for adjusting the magnitude of the wave reflection to just equal the magnitude of the wave reflection produced by the window-liquid interface. As used herein, abrupt waveguide transition means that there is a transition from one size of waveguide to a second size of waveguide which takes place over a distance measured within the waveguide which is less than firth of a guide wavelength at the center of the passband of interest.

In a preferred embodiment of the waterload 1 of the present invention, as depicted in FIG. 1, the spacing S from the window member 4 to the abrupt waveguide transition 11 is selected to be approximately 0.4)\ n is preferably as low as an integer value as possible since the bandwidth of the match produced by such a transition will vary inversely with n. In this regard, the 11:0 condition is indicated in phantom lines in FIG. 1 and corresponds to the abrupt waveguide transition 11 being moved adjacent the face of window member 4. While, theoretically, this position for the abrupt waveguide transition 11 produces the broadest possible impedance match it comlicates fabrication of the load since the precise proper position of such an abrupt transition may occur inside the window member 4 which position is extremely difficult to realize and to adjust in a practical manner.

Referring now to FIG. 6 there is shown a plot of voltage standing wave ratio versus frequency in gigahertz for the waveguide waterload 1 of the present invention as compared with the prior art waterload employing inductive matching irises. As can be seen from the plot, the bandwidth for the waterload has been increased from approximately 10% to 20% by the provision of the abrupt waveguide transition 11. This result was achieved with a spacing S equal to approximately 0.47\ and a coolant at 25 C., the coolant comprising 50% by weight ethylene glycol and 50% by weight water. The window member 4 was alumina ceramic and approximately a quarter wavelength thick. Such waveguide waterloads will operate to extremely high average powers, for example, waterloads of this type have been operated at kilowatts average power in the frequency range centered at approximately 6.1 gigahertz and have operated in the range of approximately 30 kilowatts c.w. in the frequency range centered at 24 gigahertz.

Although an odd numbered quarter wave transformer window 4 has been thus far described for matching the impedance of the waveguide 2 to the impedance of the liquid dielectric coolant, the invention is not limited to use of an odd number of integral quarter wavelengths. More specifically, if the dielectric constant of the dielectric coolant stream 5 is near 1 then a 1:1 transformer window section will provide the best match and this can be accomplished if the window member 4 is an even number of integral quarter wavelengths thick, i.e., /2. Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a waveguide load, means forming a waveguide for transmitting microwave energy from a source to a stream of dielectric liquid, means forming a microwave energy permeable dielectric window member sealed across said waveguide to provide a liquid impervious barrier thereacross, means for directing a stream of dielectric liquid over a face of said window member for absorbing microwave energy passing through said window from the source, said window member having a thickness taken in the direction of microwave power flow therethrough of approximately an integral number of quarter electrical wavelengths to provide a quarter wave impedance transformer section for matching the impedance of said waveguide to the impedance of the dielectric liquid load, microwave energy reflective means in said waveguide between the source of microwave energy and said window member for producing a wave reflection which approximately cancels a wave reflection from said window member to broaden the passband of the waveguide load, the improvement wherein, said microwave energy reflective means comprises an abrupt waveguide transition from a first section of said Waveguide having a certain cross-sectional dimension to a second section of said waveguide having a larger width and a larger height than said first section, said window member being sealed across said second section of waveguide, whereby the shape of the 6 dispersive characteristic of the wave reflection from the abrupt transition is made similar to that of the wave reflection from said window member for broadening the passband of the waveguide load.

2. The apparatus of claim 1 wherein said abrupt waveguide transition is disposed within the range of 0.375% to 0.625% from the face of said window member, where n is any integer value including zero and is the guide wavelength within said waveguide.

3. The apparatus of claim 1 wherein said abrupt waveguide transition is tapered for matching the magnitude of the wave reflection from said transition to the magnitude of the wave reflection from said window member.

References Cited UNITED STATES PATENTS 11/1966 Nelson 33322 12/1967 Johnson 333-22 US. Cl. X.R. 33334 

